1) Visually they kinda do look similar (though Kaiki is older version). Both are cynical and in their own ways find ways to manipulate things/take advantage of things to their benefit.

2) Hachiman I relate to majorly hence why I like him, especially on the 'nice girls' view of his. The world really is an unfair place and you're better off do what you have to do and not much more, while closing yourself off. I just like the idea of a non-fantasized main character that Hachiman represents, he's more realistic than some out there (not insulting those characters just pointing out that Hachiman is more realistic).

Kaiki I really don't have too much of an opinion on either way, though I dislike him hurting Karen. In a way he may even be what Hachiman has to look forward to in the future. He sees humans via a cynical lens, which imo is a good way to see them.

I guess I like seeing characters that are cynical and not saying how you should trust people and care about them cause they'll do the same for you. They won't for the majority of the time, at least that's my experience anyway.

as for Kaiki, I wanted to hate him at first because he's a con-artist but his dialogue and VA are too good!
Like, the manner in which he goes on and on about things in such a roundabout way, I love it.